laser beam machining installation comprises a laser and a machining head (also called "nozzle") located in the vicinity of the part to be machined and which focuses the laser beam.
As is known, the power density supplied by continuous CO.sub.2 lasers makes it possible to achieve the melting or volatilization temperature of any random material by simply focusing the laser beam. Thus, in addition to other applications, laser beams make it possible to perform machining processes without introducing impurities into the material to be machined or welded. Laser beam cutting processes are also particularly well adapted to the cutting and volume reduction problems occurring in a hot cell of metal parts, such as the assembly structures of nuclear fuel elements.
In the presently known welding or cutting installations, like those described for example in the article by P. R. Higginson and D. A. Campbell, "Dounreay Nuclear Power Development Establishment", the actual laser is located outside the hot enclosure and the machining head positioned within the cell focuses the laser beam onto the part to be cut. The machining head remains within the cell. Moreover, this document describes no radiation-tight sealing to ensure the confinement of radiation to the enclosure and no means are disclosed for the purpose of cooling windows or lenses placed in the path of the lser beam.
The behavior of the focussing lens of the machining head is one of the essential features occurring in an installation of this type. Thus, the machining head located within the hot cell is permanently exposed to the radiation inside the cell. Thus, under the action of this radiation, the lens becomes progressively opaque. Therefore, when traversed by the laser beam, the lens absorbs a quantity of heat, which increases as the lens becomes more opaque. The lens cooling means, provided as a function of the power of the laser beam will then become inadequate for cooling the lens. The lens becomes excessively hot leading to its fracture.
In known cutting installations, this problem has hitherto been solved by replacing the machining head after a relatively short operating period.